Oxidative stress is involved in the pathogenesis of asthma. Paraoxonase 1 (PON1) and arylesterase are esterase enzymes displaying antioxidant characteristics. PON1 activity varies widely among individuals and ethnic groups, partly related to polymorphisms. The aim of this study was to determine the activities of PON1 and arylesterase including the phenotype distribution of PON1 in asthmatic patients and healthy subjects. Forty-nine asthmatic patients and 41 healthy people were included in this study. Serum PON1 and arylesterase activities were determined by spectrophotometric assays, as well as the lipid profiles. The PON1 ratio (salt stimulated paraoxonase/arylesterase) was trimodally distributed and this ratio was used to determine the individual phenotypes of all subjects. The PON1 activity in the asthmatic patients was significantly lower (p=0.024) when compared to the healthy control group, however no significant difference in the activity of arylesterase was observed between the two groups. The prevalence of the PON1 phenotypes in the asthmatic population were 26.5%, 16.3% and 57.2 % for QQ, QR and RR, respectively. PON1 activity was significantly lower in asthmatic patients; in addition, the results of this investigation indicated that PON1 RR phenotype may be an important risk factor in asthma disease.